The present invention relates to a soft-start system in an electrical power system and in particular to the soft-start of DC links for capacitor banks of aerospace/industrial power electronics and drive systems.
Soft-start systems are used in power conversion and distribution systems to limit the in-rush currents generated therein. For example, when charging capacitor banks the initial in-rush current causes very large transient current and voltage excursions. These transient voltages and currents can over-stress semiconductors and may damage other circuit components such as DC link capacitors and Electromagnetic Interference (EMI) filter components.
FIG. 1 illustrates a conventional soft-start circuit. Traditional soft-start circuits use a resistor 110 and a switching device 120 in the DC link 180 power pass that results in significant power dissipation. Additionally, the traditional soft-start design requires a high current rating of the controlled switching device 120 because the switching device 120 must accommodate the full current supplied to the back-end inverter 170 or other load. Further, an isolated power supply is required for triggering the soft-start supply because the switch is at the same high voltage as the DC link 180.
In the circuit of FIG. 1, the soft-start system interfaces between a front-end rectifier 160 and the back-end inverter 170. The front-end rectifier 160 receives AC power from a 3 phase AC power source 150 and converts it into DC power. However, the DC power poses a problem if applied directly to the capacitor bank 130 because the capacitor bank appears as a short circuit when the DC voltage is first applied. Therefore, a current limiting resistor 110 is inserted into the circuit to limit the in-rush current when the DC voltage from front-end rectifier 160 is first applied. After the capacitor bank 130 is charged, the current draw from the capacitor bank essentially becomes zero. Additionally, the voltage across the DC bus 182, 184 approaches the output voltage of the front-end rectifier 160. The triggering circuit 140 detects this voltage and triggers the switching device 120 when the voltage reaches a predetermined threshold. Switching device 120 when activated shorts out resistor 110. However, as noted above, this disadvantageously place switching device 120 in series with the back-end inverter, which results in switching device 120 having to conduct the full current drawn by the back-end inverter. Thus, the switching device must be sized for this full current rating. Typically, the switching device 120, heat sink (not shown) and related components will be relatively large and expensive.
FIG. 1 shows a conventional soft-start system that is used for preventing excessive current/voltage excursions during the initial starting of a power electronic system using a DC link. This design requires a high voltage and high current switching device with excessive power dissipation during normal operation. Alternatively, a relay can be used that is very expensive and bulky due to the high current and high voltage rating of the relay. Therefore, this soft-start system is difficult to implement and requires a large physical envelope for the components.
Therefore, it is desired to have a soft-start system that does not require the switching device, either solid state or electromechanical relay, to continuously carry the full current of the power system
In accordance with the present invention, the deficiencies in prior systems are overcome by providing a soft-start device for electrical power systems that has the switching device removed from the DC link power pass. According to the present invention, the switching device and resistor are placed in series with a capacitor bank out of the DC link power pass. According to an embodiment of the present invention, the soft-start system includes: a rectifier that receives AC power from a source and converts the AC power into DC power in a DC link; a capacitor connected to a first bus of said DC link; a resistor connected a second bus of said DC link, wherein said resistor and capacitor are connected in series; a switching device connected in parallel with said resistor; and a triggering circuit for measuring a DC voltage on the DC link and activating the switching device to short circuit the resistor.
According to another embodiment of the invention, a method for soft-starting a DC link in an electrical power system comprises: charging a capacitor connected to a first bus of the DC link, wherein a resistor is connected to a second bus of the DC link, and wherein the resistor and capacitor are connected in series; measuring the charge of the capacitor; and activating a switching device, wherein the switching device is connected in parallel with the resistor, and wherein the switching device when activated short circuits the resistor.